Oceanographers observed changes in the chemistry of surface water in the Bay of Bengal over three surveys taken in 1994 (gray circles), 2002 (black circles), and 2006 (open circles). They linked these changes to the loss of Himalayan glaciers: Meltwater, low in oxygen-18, flows from the glaciers through the Ganga-Brahmaputra river system into the bay.

Credit: Environ. Sci. Technol. Lett.

Seawater Survey

Oceanographers observed changes in the chemistry of surface water in the Bay of Bengal over three surveys taken in 1994 (gray circles), 2002 (black circles), and 2006 (open circles). They linked these changes to the loss of Himalayan glaciers: Meltwater, low in oxygen-18, flows from the glaciers through the Ganga-Brahmaputra river system into the bay.

Credit: Environ. Sci. Technol. Lett.

Many glaciers around the world are receding as global temperatures rise. Scientists want to estimate how fast they are disappearing, but, in the Himalayas, this can be a difficult task. The region’s treacherous terrain and high altitudes make field measurements challenging.

Now, oceanographers report a new strategy to track the melting of Himalayan glaciers: measuring its imprint on seawater (Environ. Sci. Technol. Lett. 2014, DOI: 10.1021/ez500076z). They have estimated glacier loss in the Himalayas based on changes in the salinity and oxygen isotope composition of the Bay of Bengal, which receives most of the region’s glacial meltwater.

Oceanographer Arvind Singh and his colleagues at the Physical Research Laboratory in Ahmedabad, India, developed the method after noticing unexpected results from three seawater surveys of the Bay of Bengal taken between 1994 and 2006. They found that the linear correlation between the salinity and oxygen-18 content of surface seawater—a relationship that is usually constant—had steadily changed over those surveys.

To explain this change, Singh, who is now a postdoctoral fellow at GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, and his colleagues made a model of the bay’s inputs and outputs. The inputs included the amount and isotopic composition of rainfall and discharge from the Ganges and Brahmaputra Rivers, which carry glacial meltwater to the bay. Based on the model and observations, the scientists concluded that water from the melting glaciers best explained the changes in the Bay of Bengal. Besides having low salinity, glacial meltwater also has low oxygen-18 content. It is low in this heavy isotope because by the time water vapor turns into snow at high altitudes, much of the oxygen-18 has preferentially rained out at lower altitudes.

Using their model, the team estimates that Himalayan glaciers lost mass at 20 km3/year between 1994 and 2006. The value is greater than estimates based on regional field measurements, but within the range of recent satellite-based estimates of between 14 and 38 km3/year (Science 2013, DOI: 10.1126/science.1234532). The new estimate is more precise than the satellite-based ones, with less than 10% uncertainty.

If this loss rate continues, the researchers project that glaciers in the Himalayas could disappear in 200 to 400 years.

The method is ideal for bodies of water like the Bay of Bengal because it has a stable water column with little mixing of deep water, which has a different chemical signature from surface water, Singh says. Thus it would be difficult to use the technique to estimate glacial loss in more turbulent regions, such as the waters around Greenland or Antarctica.

Graham Cogley, a glaciologist at Trent University, in Canada, says the method is intriguing. “It’s a potential new way of finding what’s happening to the glaciers very indirectly.” But he cautions that the team’s projection for the glaciers’ disappearance is crude and of little value, because it assumes recent loss rates will remain constant over time. “There is simply too much that each generation can do, or not do, to change things” that would affect the climate, he says.